A good example of a fluidized particle contacting process is the fluidized catalytic cracking of hydrocarbons. The fluidized catalytic cracking of hydrocarbons is the mainstay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains or ring structures thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed. The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's and, along with its method of operation, is well known to those skilled in the art of hydrocarbon processing.
The cracked products from an FCC reaction section are first separated from the particulate material by disengagement in a reactor vessel or by any other primary separation device followed by passage of the vapor stream through at least one secondary separator to remove the majority of any entrained particulate material. The separated vapors are then delivered directly to product separation facilities associated with the FCC unit. These separation facilities include a primary separator, often referred to as a main column, and a compression section containing numerous separators and contactors for further separating overhead vapors from the main column. The compression section is commonly referred to as the gas concentration section. Invariably the vapors passing to the product separation facilities will contain a small quantity of the most fine particulate material that also enters the product separation facilities.
Routinely in the prior art, as shown by U.S. Pat. Nos. 3,849,294; 3,458,691; 4,003,822 and 3,042,196, the primary separator or the main column separates the remaining heavier fractions into product streams such as gasoline and other distillates, into other heavier streams for recovery and/or other processing such as light cycle oil and heavy cycle oil, and into a bottom stream that is ordinarily recycled to the reaction zone. Entrained fine particles collect in the heavy bottom stream. As shown by the above-cited references, a settler ordinarily concentrates the catalyst particles into a slurry that also passes back to the reaction zone. The return of the solids concentrated from the main column bottoms in a separator or other device tends to increase the concentration of solids in the circulating hydrocarbons that circulate in a recycle loop from the reactor through the main column bottom and back to the reactor. The solids eventually escape from the reactor recycle loop by passing in small quantities through the stripper and finally to the regenerator. The most fine particles tend to remain confined in the circulation loop on the reactor side of the process due to the tendency of the lighter particles to remain with the products carried overhead by the reactor cyclones. This type of circulation can result in solids equilibrating in the reactor--main column recycle loop of the process and causing a threefold increase in the solids concentration before the trapped fine particles exit the process via the regenerator and flue gas system. The three pass average for the circulation of fine catalyst particles through the slurry circuit before escaping the process aggravates erosion and plugging problems in the slurry circuit and often overloads any filtration systems that employed to concentrate the solids for recovery and recycle. Today's practice of closing the cyclone and other reactor systems for vapor containment the problems of excessive fine particle recycle in the slurry system by the increasing concentration of solids in downstream cycle separators.
Other prior art systems have been known that recover the fine catalyst particles in a different manner for return to the reaction side of the process. U.S. Pat. No. 2,859,175 shows a system wherein the solids are recovered from a main fractionator and passed back to the top of a dense bed that holds catalyst for passage to a reaction zone. The '175 system provides no way for the fine particles to escape from the dense bed that supplies the catalyst to the reaction zone without first passing the fines again through the fractionator. Some of the very early FCC U.S. Pat. No. 2,687,988 did not need to consider the recirculation of fines in any manner separate from the general recirculation of the catalyst.